1. Field of the Invention
The present invention relates to an LED power supply device. The power supply device of this invention is suitable for in-vehicle use.
2. Description of the Related Art
In an LED power supply device whose conducting current is a few tens of milliamperes, current applied is controlled by connecting a resistor in series with the LED. However, as LED brightness is required to be higher so that the conducting current is a few hundreds of milliamperes, heat released and power consumed by the resistor are innegligible. In addition, since resistor size is larger, and design for heat releasing on a substrate side is required, constraints on design in portions other than the power supply circuit of the LED are caused.
Such resistor constraints can be overcome by using a switching power supply circuit. FIG. 1 shows an example where a versatile switching power supply circuit is applied to an LED.
In this circuit, feedback voltage from the LED is compared with reference voltage from a reference voltage generation circuit 3 at an error amplifier 2. A high-precision circuit of bandgap reference type is typically used for the reference voltage generation circuit 3. A result compared is amplified by a comparator 4, and compared thereat with oscillating voltage of a triangular wave oscillator 5, thereby opening/closing a switching circuit 6. Such versatile switching power supply circuit 7 is typically an integrated circuit.
Japanese patent application laid-open No.2002-98375 discloses such a versatile switching power supply circuit.
The power supply circuit shown in FIG. 1 is versatile, and not specially designed for the LED. It has unnecessary functions for LED lighting control. Although it is equipped with, for example, the independent oscillator 5 so as to continue voltage control operation irrespective of load presence/absence and kind, the oscillator 5 is an unnecessary function in the case where the load is limited to the LED. Although the power supply circuit shown in FIG. 1 uses the high-precision bandgap reference type reference voltage generation circuit 3 with very high temperature dependence to maintain high-precision voltage control, the high-precision bandgap reference type reference voltage generation circuit 3 is also an unnecessary function in the case where the LED is controlled. This is because even if current applied to the LED is changed by about ±20% in a high-brightness region, the human eye cannot recognize its change.
Since the reference voltage of the bandgap reference type reference voltage generation circuit 3 used in many cases is set to more than 1.25V, the feedback voltage from the LED 1 is more than 1.25V. As a result, power consumption due to feedback loss is innegligible.
A switching power supply IC for an LED only has been placed on the market. However, such ICs are costly because they use high-speed switching device for miniaturization, or specialized circuit configuration for high efficiency.